System for addressing a destination station using only address of a network junction and station link identifier in a network with plurality of segments

ABSTRACT

A process and a device for addressing stations in a network consisting of at least two segments (S1, S2, S3), wherein network junctions (N1, N2) are provided between the individual segments, which network junctions forward a message from a first segment (S1) to other connected segments (S2, S3) on the basis of path information contained in messages for addressing. The path information of a message to be forwarded through a network junction (N1) contains the station address, valid in the first segment (S1), of the network junction (N1) and an address extension containing a station link identifier for the network junction (N1), and with the help of said address extension the network junction (N1) inserts path information, valid in the other segment (S2), in the message for addressing the receiving station (E2, E3) and forwards it to the other segment (S2). The path information for each station link that can be established via a network junction are advantageously stored in a transmission table. Such transmission tables can be generated automatically by a special procedure in the network junctions.

FIELD OF THE INVENTION

The present invention relates to a process and a system for addressingstations in a network which contains two or more segments.

BACKGROUND INFORMATION

The transfer protocol of PROFIBUS, capable of being connected to anetwork consisting of a plurality of segments, is described in DIN19245, Part 1. Network junctions connecting the segments are providedbetween the individual segments to enable stations of different segmentsto communicate. In order to address the receiving station, messagescontain path information providing at least the address of the receivingstation. At this address, the receiving station identifies a messageaddressed to it. The address can be an individual address for addressinga single station or a collective address for addressing several or allmembers in the system. Each station has an individual address, which isunique to the particular segment to which the station is connected.

German Published Patent Application No. 40 00673 Express Mail No.: EM271950450 US describes inputting the station address of the receivingstation, which is valid in that station's segment, as well as thesegment addresses of the segments through which the message is to beforwarded. According to this related art, a transmitting station wishingto communicate with a receiving station must have the requiredinformation concerning the full message path saved in a storage deviceand insert this information in the message. For each message, networkjunctions must check whether the segment numbers contained in the pathinformation agree with the numbers of the segments they connect and, ifso, forward the message. The disadvantage of this related art is thatthe number of segment addresses in the path information increases in thecase of large networks according to the number of segments involved inthe transmission, and therefore the useful data length to total messagelength ratio decreases. Furthermore, considerable storage space isrequired for storing the extensive path information for all stationlinks in each station. To remedy this problem, the aforementioneddocument proposes including only the station address and station segmentnumber in the path information when addressing a receiving station. Thenetwork junctions then carry a list of segments that can be reachedthrough them and only forward a message if the number of a reachablesegment is contained in the path information. While this process reducesthe amount of data required for the path information of the messages,problems may arise if the network structure changes or a receivingstation is moved to another segment.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process foraddressing stations in a network consisting of at least two segments,wherein a favorable useful data length to total message length ratio isachieved and wherein the segment number of a receiving station does notneed to be transmitted even for messages transmitted over a plurality ofsegments, as well as a device suitable for carrying out the process.

According to the present invention, a message intended for a station inanother segment is addressed by the transmitter of the message to thecorresponding network junction connected to the same segment with astation link identifier for the network junction. The network junctioncan then determine the address to which the message is to be forwardedthrough the station link identifier with the help of the informationstored in the network junction and it can forward the message. Ifanother network junction is located in the path, it is addressed, i.e.,the same principle is used again. The message is then provided with thenew destination address in the network junctions and transmitted to thenext network junction or from the last network junction to the receivingstation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a network with three segments, in accordance with thepresent invention.

FIGS. 2 shows the possible station links for a network junction, inaccordance with the present invention.

DETAILED DESCRIPTION

In a network according to FIG. 1, a station E1 is located in a segmentS1, connected to a second segment S2 via a network junction N1. SegmentS2 is in turn connected to a third segment S3, where station E3 islocated, via network junction N2. A station E2 is connected to SegmentS2. If station E1 (transmitting station) wishes to transmit a message tostation E3 (receiving station), it must know the identifier of thecorresponding station link and that E3 can be reached via networkjunction N1. Therefore, E1 addresses N1 in segment S1 and transmits thestation link identifier to N1 in a path information string. Theinformation that the message of this station link must pass through N2and the station link identifier for N2 is stored in N1. Therefore itinserts this identifier in the message and addresses the message insegment S2 to network junction N2. Network junction N2 finallyidentifies the address of receiving station E3 for this station link,since station E3 is connected to the same segment, and forwards themessage to it. It is advantageous that each station be capable ofaddressing one or more other station in another segment without knowingtheir location in the network or their addresses in the segment. Eachtransmitting station should only know through which network junction therespective station(s) can be reached, i.e., the address of this networkjunction in the station's own segment and the station link identifier.Another advantage is that, from the transmitting station's point ofview, the functionality of the receiving station seems to reside in thenetwork junction located in its own segment. The station link identifieris comparable to a service access point known from DIN 19245, Part 1(LSAP-Link Service Access Point) and can be transmitted accordingly. Thefunctions concentrated in the receiving stations can be called through anetwork junction using the station link identifier, as in receivingstations where different services or functions of the receiving stationcan be addressed through the address extension with an LSAP. To thetransmitting station, these services seem to be concentrated in thenetwork junction. Therefore each station only needs to know theaddresses of the other stations, including the network junctionsconnected to its own segment. Knowledge of the structure of the entirenetwork or of the segments located beyond a network junction is hiddento the station and is not needed.

Yet each network junction, when it receives a message, must know where,i.e., to which address, and with what station link identifier it shouldforward it. For this purpose each network Junction needs a memory areafrom which the receiving station address, valid in the other segment,can be obtained for each station link. This memory area will behenceforward referred to as a transmission table. The exact format ofthe transmission table is not established. If a transmitting stationwishes to communicate with a plurality of receiving stationsconsecutively, the network junction maintains a plurality of stationlinks for this transmitting station. The transmission table contains anentry for each of these station links.

FIG. 2 shows an example of a network junction N3 that carries aplurality of station links. A segment S4 with a station E4 is connected,via network junction N3, to a segment S5, to which stations E5 and E6are connected. A communication channel is set up in network junction N3for each possible station link. A message from station E4 (transmittingstation) to station E5 (receiving station) is first transmitted insegment S4 to network junction N3, which has the address 21 in segmentS4. At the same time, a station link identifier K1 is transmitted tonetwork junction N3. Using the transmission table, where address 9 ofreceiving station E5 in segment S5 is stored, the destination address ofthe message in segment S5 is obtained, and the message is forwarded.Station E4 also addresses a message intended for station E6 to networkjunction N3 with the address 21 in segment S4, however, it inserts K2 init as the station link identifier. According to another entry in thetransmission table for this communication channel, network junction N3forwards this message to address 16 of receiving station E6 in segmentS5. If, conversely, station E5 wishes to transmit a message to stationE4, it addresses the message to address 3 of network junction N3 insegment S5 and communicates to it station link identifier K3. Networkjunction N3 finds the address 7 of receiving station E4 under identifierK3 and transmits the message to it. The transmission of a message fromtransmitting station E6 to receiving station E4 takes place in a similarmanner using station link identifier K4.

Groups of stations or all the other stations can be receivers of amessage.

The present invention has the advantage that, contrary to the knownsolutions, the load on stations E1, . . . , E6 is reduced as they do notneed to store path information in the form of lists of segment addressesfor the links to other stations. Neither do they need any information onthe structure of the network. This information is distributed over thenetwork and it is available only where it is needed, namely in thenetwork junctions N1, . . . , N3. The resources used by stations E1, . .. , E6 are also reduced by the fact that they do not need to distinguishbetween intrasegment communications, i.e., those taking place in thesame segment, and extrasegment communications, which occur acrosssegment boundaries. In previous processes, where stations insert thesegment addresses of the receiving station in the path information of amessage, stations set up for communication in a single segment only andincapable of working with segment addresses, must be either upgraded,for example, through reprogramming, or replaced with new machines whenthe network is expanded to contain more segments, so they cancommunicate with stations in other segments. In the process according tothe present invention, this is not necessary. A station enabled to havean LSAP address extension needs no function upgrade for communicationacross segment boundaries. The effects of address or network structurechanges, e.g., changes in segment numbers, additions of new segments, ormoving a station to another segment, are limited to the immediateenvironment, i.e., to the affected segments. Neither do such changesaffect the stations in the other segments. In the known processes thatwork with segment addresses in the path information the segmentaddresses or station addresses for addressing receiving stations mustalso be changed in the stations located in other segments thatcommunicate with the directly affected stations. The present inventioncan also be used in networks consisting of segments of different types,since adjustment to another type of communication may take place in thenetwork junctions. The properties of the different subnetworks areisolated through the network junctions. In addition, using theaddressing according to the present invention, less resources are usedin the stations, since the station must only evaluate, in a messageinput filter, the addresses that are valid in the given segment in orderto determine whether the message is intended for it. Other addresses,such as segment addresses, do not need to be evaluated.

The transmission tables can basically be prepared manually and loaded inthe network junctions, but it is also possible to generate themautomatically in the network junctions using the following method. Oneprerequisite for the method is that each station must have an identifierthat is unique in the network. Let us assume, for example, that astation link is to be established between station El, representing acontrol system L1 and station E3, a temperature sensor S11, in thenetwork of FIG. 1. The control system L1 needs the temperature measuredby sensor S11 for its control function. For this purpose, station E1transmits a special search message in its own segment S1 containing, inaddition to its own transmitter address valid in segment S1, thedesignation S11 of the temperature sensor sought and an identifier as asearch message. This message is evaluated first in the stationsconnected to segment S1, including the network junctions (for the sakeof simplicity represented by network junction N1 in FIG. 1). The stationwhose identifier agrees with the identifier transmitted responds to thesearching station E1, which means none in this case, since station E3 isnot connected to segment S1. A search message must first pass throughthe network junctions connected to the segment, so it can reach all thestations in the network. Since at this point only some transmissiontables exist or none at all, the message goes through a specialcommunication channel, which is operational from the very beginning andrequires no entry in the transmission table. Thus, in the presentembodiment, network junction N1 forwards the search message transmittedby E1 to segment S2. At the same time, it temporarily stores the addressof station E1, valid in segment S1, for a predefined time period andinserts its own address valid in segment S2 in the search message.Station E2 processes the search message, but it does not find itsidentifying designation in it, so it does not respond. Network junctionN2 inserts its address, valid in segment S3, in the message again,transmits it to segment S3 and temporarily stores the address of networkjunction N1 valid in segment S2. Station E3 finally receives the searchmessage and recognizes its identifying designation S11 in it. Ittransmits a response message to network junction N2 with its addressvalid in segment S3. Upon receipt of the response message, networkjunction N2 enters a new station link identifier and inserts inpathinformation, which requires an address valid in segment S3 of stationE3. At the same time, it transmits a response message containing itsaddress valid in segment S2 and the new station link identifier tonetwork junction N1 via segment S2. Network junction N1 also enters anew station link identifier in its transmission table and inputs thedata received from network junction N2. Finally, network junction N1transmits a response message to the seeking station E1, which containsits station link identifier and its address valid in segment S1. As theresponse message is received by station E1, the transmission informationhas already been stored in all the network junctions involved, i.e., thepath is open for communication. This process is repeated for eachrequired link between the network stations. In this way, all thetransmission tables required in the network junctions are generatedautomatically.

This process advantageously reduces the resources used by the stationsfor establishing a station link. If the transmission tables were firstgenerated in an organization system on the basis of the responses andthen loaded into the network junctions, a considerably greater storagespace would be needed in the organization system. In the novel process,this need no longer exists, since only the information of the connectedsegments that is relevant for the specific station link (but not thetransmission tables) for the entire network is needed. The time requiredand the network load are also reduced, since the transmission tables aregenerated automatically in the network junctions and do not need to beretransferred there. The novel process also implies simplification ofthe network junctions, since no transfer procedures are needed for thetables.

Contrary to a process where the table is loaded, in the process of thepresent invention, the format of the table does not need to be exactlyestablished. The tables are generated locally in the network junctions;therefore the process is advantageously more flexible when the networkjunctions connect segments of different types, for example, segmentswith different address structures. Path information with differentstructures, optimized for the properties of each segment, can be enteredinto the transmission tables for different directions of transmission.In this case it is advantageous if a separate transmission table is setup for each direction of transmission.

What is claimed is:
 1. A process for addressing stations in a networkcontaining at least two segments and network junctions arranged betweenthe at least two segments, wherein the network junctions forward amessage from a first of the at least two segments to another of the atleast two segments on the basis of path information contained in anaddressing message, the process comprising the steps of:forwarding amessage through one of the network junctions, the message including pathinformation containing a station address of the network junction and anaddress extension, wherein the station address is valid in the firstsegment and the address extension includes a station link identifier,which address extension uniquely identifies for the network junction alink that is to be established in the network between a transmittingstation and a receiving station via the network junction; forwarding,from the network junction, the message to another segment if the stationaddress of the network junction agrees with the received station addressin the path information; and updating, at the network junction inaccordance with the station link identifier, the path information of themessage for addressing the receiving station, wherein the pathinformation is valid in the other segment.
 2. The process of claim 1,wherein the one network junction obtains the path information which isvalid in the other segment from a table which includes an entry for eachlink that can be established via the network junction.
 3. The process ofclaim 2, wherein each station in the network has a unique identifyingdesignation, and wherein:to establish a new station link, a transmittingstation transmits a search message containing the address of thetransmitting station valid in the first segment and an identifyingdesignation of the receiving station, each station evaluates the searchmessage and determines whether the identifying designation in the searchmessage agrees with the identifying designation of the respectivestation; the network junction stores the identifying designation of thestation emitting the search message in the first segment and forwardsthe search message to the other segment, inserting in the search messagethe address valid in the other segment, a receiving station whoseidentifying designation agrees with that of the one received transmitsback to the transmitting station a response message, which is to be usedfor the station link, with the address of the receiving station which isvalid in the respective segment; and a network junction receiving aresponse message stores an entry with the valid path information foraddressing the receiving station as a new station link identifier in thenetwork junction's table, and forwards the response message with itsaddress valid in the respective segment and the new station linkidentifier.
 4. A network comprising:at least two segments; and a networkjunction between each of the segments, which network junction forwards amessage from a first segment to a second segment on the basis of pathinformation contained in the message, wherein:the path information ofthe message to be forwarded through a network junction contains astation address of the network junction and an address extension,wherein the station address is valid in the first segment; the addressextension contains a station link identifier uniquely identifying forthe network junction a link that is to be established between atransmitting station and at least one receiving station in the networkvia the network junction; and the network junction is configured so thatit forwards the message to another segment if its station address agreeswith the received station address in the path information and, on thebasis of the station link identifier, updates the message's pathinformation, valid in the other segment, for addressing the receivingstation.